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Abstract

Yams (Dioscorea spp.) belonging to the family of the Dioscoreaceae was of considerable economic importance in Puerto Rico in the 80’s agricultural production. Since the 90’s its local production has been dramatically declining due to different pests and diseases, economic constraints and high production costs. The lack of effective fungicides has limited farmers to manage foliar diseases such as anthracnose and other foliar diseases in yam. Thus, the objectives of this research were: 1. To evaluate in vitro and in vivo organic and conventional fungicides to manage foliar diseases in yam, 2. To identify the causal agent of the yam dieback in the cultivar Florido and 3. To evaluate the sensitivity of two Colletotrichum isolates to azoxystrobin. The fungicides selected for in vitro and in vivo experiments were Switch®, Trilogy®, Regalia®, Kphite®, Chlorothalonil®, Fontelis®, Kocide® and Quadris®. The latter being the only foliar fungicide registered to be used in yams in Puerto Rico. A colorimetric assay was used in the in vitro phase to evaluate the efficacy of fungicides. The field experiment was established at the Agricultural Experiment Station in Isabela and the design used was a randomized complete block with four blocks and ten treatments. For the study of the etiology of stem dieback in the cultivar, Florido symptomatic samples were collected in the yam storage facilities at the Agricultural Experiment Station in Isabela. With the isolates obtained, a pathogenic, morphological and molecular characterization was performed. The evaluation of the sensitivity of Colletotrichum isolates to azoxystrobin was conducted using a PDA plates amended with the fungicides with eight treatments and four replications. In vitro, Chlorothalonil® 720, Switch® and Fontelis® were the most effective fungicide inhibiting the mycelial growth of C. gloesporioides and C. alatae. In field experiments, the most effective fungicides for managing foliar diseases of yam including anthracnose was Switch®. None of the plant-based extracts (Trilogy® and Regalia® SC) and the other conventional fungicides evaluated provided a significant reduction in disease severity. Eight fungi were isolated from the symptomatic stem of Dioscorea alata cv. Florido. Six of these fungi resulted to be pathogenic in the Koch’s postulates. The identified fungi combining morphology and phylogenetic analysis were Colletotrichum queenslandicum, Fusarium oxysporum (2 isolates), Fusarium sp. and two isolates of the Botryosphaereaceae family. Colletotrichum queenslandicum was the most virulent isolate causing lesions three days after of the initial inoculation. In the evaluation of the sensitivity to azoxystrobin, both isolates (C. alatae and C. gloesporioides) grew in the amended media with the fungicide similar to the non-amended treatment. These results suggest the inheritance development of resistance by Colletotrichum isolates exposed to azoxystrobin in the island. All of the results presented in the present research provide a significant advance in the evaluation of the efficacy of OMRI-certified fungicide and conventional fungicides to manage important foliar diseases of yam in Puerto Rico. Also, resistance of Colletotrichum isolates to azoxystrobin could be high to moderate and resistance management should be considered.